I love Statham as an action hero, but he's still relatively new-ish. His repertoire isn't nearly as comprehensive to be compared with Arnold. In this debate I would go with Arnold, without hesitation.
But like others have said, Willis would have been a much better contender in a round against Arnold. Now that would be hard for me to decide... but if you compare them movie-wise, I think Arnold would have a slight advantage. My debate would center more on action hero with range, which is Statham's problem, because he's The Transporter in every movie he's in.
Last Action Hero, Kindergarten Cop, Terminator 2, True Lies, Jingle All the Way... I can watch those bad boys on a loop. Then again, Bruce Willis in Die Hard, Last Man Standing, The Fifth Element, Pulp Fiction... that is a tough call.